CONDYLARTHRA. 403 



Periptychus carinidens Cope. 



American Naturalist, 1881, March, p. 337. Paleontologlcal BuUetiu No. 33, p. 484, Sept. 1881. 

 Plate XXV n, fig. 16; XXIII <i, figs. 14-15; XXIV j, fig. 5. 



This species is, with my present knowledge, only distinguished from 

 the P. rJiahdodon by its inferior size and its longer and narrower last infe- 

 rerior molar. This difference is seen in both adults and young of corre- 

 sponding ages, and chiefly in the true molar teeth. While the length of 

 those of the lower jaw varies in the P. rhabdodon from 33 to 35 millimeters, 

 in the P. carinidens it only reaches 29 millimeters, or with the posterior two 

 premolars .'iO millimeters. The details of the adult dentition do not differ 

 from those of the P. rhabdodon, except that the last inferior molar is nar- 

 rowed and produced posteriorly. 



The permanent dentition of this species is represented on Plate XXIII (Z, 

 figs. 14-15; and the milk dentition on Plate XXV a, fig. 16. As the species 

 was established on the latter specimen, I give a description of it. 



The second and third milk molars support a principal median cusp, a 

 broad heel, and a prominent anterior cingulum. The heel is more or less 

 divided into tubercles; the anterior cingulum is on the inner side, and rep- 

 resents the anterior cusp of a sectorial tooth. On the inner side of the 

 principal cusp a cingulum rises, forming a flat internal tubercle. The heel 

 of the second supports three tubercles, of which the external is the largest. 

 The anterior cingulum supports a small cusp and then rises to the internal 

 tubercle, which is compressed. The sides of all the cusps are marked with 

 distinct, well-separated vertical ridges. Each extremity of the internal cusp 

 is connected with the principal cusp by a ridge. The first true molar has 

 few cusps. Those of the heel are scarcely distinct, and form a border 

 which rises prominently into the flat internal tubercle, which forms a nar- 

 row longitu-dinal blade. The anterior cingulum has no cusp, and does not 

 rise into the inner tubercle. The principal cusp has a strong entering 

 groove next the inner tubercle. 



In these deciduous teeth the compressed semisectorial character seen 

 in the first inferior true molar of Haploconus is carried back to the second 

 true molar. The sectorial character is increased by the heel-like cliaracter 

 of the posterior front of both first and second true molars, which lacks the 



